1. Field of the Invention
The present invention provides a driving circuit of a fan, and more particularly, a driving circuit for decreasing noise and for protecting a motor of the fan.
2. Description of the Prior Art
Computers have become the most important information hardware of modern times. In general, the higher the processing speed of a computer system is, the higher the temperature in the computer system will be (especially from heat generated by a central processing unit—CPU). Only if heat of the computer system can be effectively dissipated, can the computer system operate properly and stably. Therefore, radiator fans are set in the computer system for cooling down the CPU, a graphics card, etc. Although the radiator fans can dissipate heat in a case of the computer system to the outside of the case, noise generated by the rotating radiator fans is annoying. In order to suppress the noise, the prior art provides a driving circuit using a technology of pulse width modulation, or PWM, to control rotational speed of a radiator fan based on detected temperature.
Please refer to FIG. 1, which illustrates a schematic diagram of a prior art driving circuit 10 of a radiator fan. The driving circuit 10 drives the radiator fan using PWM technology, and includes a magnetic pole sensor 100, a timing controller 102, a triangular waveform generator 104, a control signal generator 106, and a driving stage 108. The magnetic pole sensor 100 can be a Hall sensor, utilized for sensing the magnetic pole of a rotor of the radiator fan, and transmitting the magnetic pole sensing signal to the timing controller 102. The control signal generator 106 generates a control signal VPWMC according to a triangular signal VTRI generated by triangular waveform generator 104 and an external signal VPWM. The timing controller 102 sequentially transmits the control signal VPWMC to gates of transistors V1P, V1N, V2P, and V2N in the driving stage 108 according to the magnetic pole sensing signal generated by the magnetic pole sensor 100. The driving stage 108 can be a full bridge driving circuit, and can output currents through different ways from terminals O1 and O2 to a coil 110 of a stator of the radiator fan by switching the transistors V1P, V1N, V2P, and V2N. Then, by electromagnetic induction, silicon steel sheets wound around by the coil 110 generate different magnetic poles, so as to push the rotor to rotate.
In order to change rotation speed of the radiator fan, the driving circuit 10 can adjust a duty cycle of the control signal VPWMC through the signal VPWM according to temperature. Please refer to FIG. 2, which illustrates a schematic diagram of waveforms of signals VTRI, VPWM, and VPWMC in FIG. 1. As shown in FIG. 2, the signal VPWM is used to adjust the duty cycle of the control signal VPWMC, so as to control the duration of outputting current and control the rotation speed of the radiator fan. When temperature is high, the driving circuit 10 can operate the radiator fan in a full speed mode, meaning that the driving circuit 10 outputs full-wave current to the coil 110. Oppositely, when temperature is low, the radiator fan is operated in a variable speed mode. Please refer to FIG. 3 and FIG. 4, which illustrate schematic diagrams of waveforms of the terminals O1 and O2 corresponding to the variable speed mode and the full speed mode. As shown in FIG. 3, when operating in the variable speed mode, the driving circuit 10 can adjust duty cycles of signals VO1A and VO2A of the terminals O1 and O2 to control the rotation speed of the radiator fan. However, since frequencies of the signals VO1A and VO2A are fixed, it is easy to generate high-frequency noise when changing the rotation speed of the radiator fan. Furthermore, in FIG. 4, when the driving circuit 10 changes to the full speed mode, signals VO1B and VO2B of the terminals O1 and O2 generate impulse, or peak, voltages, which cause high-frequency noise, damage the driving circuit 10, and reduce the lifetime of the radiator fan.